(1) Field of the Invention
The present invention relates to a cathode ray tube. More particularly, the present invention relates to an internal magnetic shield within the improved cathode ray tube having a shape adapted for correcting the trajectory of electron beams that have been deviated by an external magnetic forces, most typified by terrestrial magnetism.
(2) Description of Related Art
FIG. 11 shows a conventional cathode ray tube (hereinafter referred to as CRT) used as a TV display or a monitor for a personal computer. An electron beam 111 emitted from an electron gun is deflected vertically or horizontally by a deflection coil 112 to scan the whole screen to reproduce an image. In this process, what is called landing deviation may happen. That is to say, the electron beam 111 may deviate from an intended trajectory and may not reach an aimed position when it receives an external magnetic field such as terrestrial magnetism from a direction perpendicular to the direction of the trajectory of the beam. In FIG. 11, the center line indicates an originally intended trajectory, and the center line the deviated trajectory (this example is exaggerated a little). To prevent the landing deviation, an internal magnetic shield 115 is disposed to surround a path of the electron beam inside the CRT (in this case, inside a funnel). It should be noted here that in the CRT, typically a raster scan method is used. In the raster scan method, the deflection coil controls the amount of deflection of the electron beam so that the electron beam horizontally scans the screen (toward the front or back in FIG. 11) and scans vertically (in the direction of Y in FIG. 11), the horizontal and vertical scans forming a raster.
The internal magnetic shield 115 cannot shield the CRT completely from external magnetic fields. Therefore, in reality, the internal magnetic shield 115 (a) shields the CRT from external magnetic fields to some extent, (b) changes the direction of the magnetic force so as not to affect the election beam, or (c) corrects the force the electron beam receives at a certain position.
Except for some special cases, an external magnetic field that affects the electron beam is the terrestrial magnetism. The terrestrial magnetism is divided into a horizontal component (a horizontal component of a vector relative to the viewing angle of the screen) and a vertical component (a vertical component of a vector relative to the viewing angle of the screen). As is well known, the vertical component changes the landing uniformly over the whole screen. The landing deviation caused by the vertical component are not regarded as a problem since the phosphor screen position is corrected using a correction lens or the like when the phosphor screen is formed.
In contrast, a horizontal magnetic field changes its direction depending on the relative positions of the CRT and the magnetic field. Typically, as shown in FIG. 12, a horizontal magnetic field 120 is divided into a CRT tube-axis direction 121 and a lateral direction 122. Here, the space through which the electron beam passes is conical, expanding as the electron beam proceeds. The axis of the conical space through which the electron beam passes is called tube axis.
To achieve a shield from the terrestrial magnetism, it is necessary to consider the magnetic characteristics of a lateral magnetic field and a tube-axis-direction magnetic field which are components of a horizontal force of the terrestrial magnetism.
It is possible to apply from outside a magnetic field force equivalent to the terrestrial magnetism force or stronger, measure the electron beam landing deviation on the phosphor screen caused by the application of the force, and evaluate the magnetic characteristics in the CRT. FIG. 13 shows the measuring points: four corner points; and two center points (hereinafter referred to as NS points) of upper and lower portions. Here, important characteristics are as follows:
(1) characteristics at corner points (hereinafter called lateral characteristics) when a lateral magnetic field is applied; and
(2) characteristics at NS points (hereinafter called NS characteristics) when a tube-axis-direction magnetic field is applied.
FIG. 14 shows the shape of a conventional internal magnetic shield 115. The internal magnetic shield 115 generally has a truncated, open pyramid shape including two long sides 141 opposite to each other and two short sides 142 opposite to each other, where an opening 143 formed at the top and an opening (not shown) formed at the bottom.
Recently, CRTs with a large-screen or a flat-screen are becoming mainstream. A conventional shadow mask used in a flat-screen CRT is typically manufactured by tightly spanning a plurality of wires between opposite sides of a frame.
In CRTs with such a conventional internal magnetic shield, the landing deviation caused by terrestrial magnetism tends to increase. This is because, with the conventional shadow mask, the magnetic reluctance of the shadow mask generates undesired magnetic field in the vicinity of the shadow mask (R. Murai et al., xe2x80x9cHome Base Shaped Inner Magnetic Shieldxe2x80x9d SID2000DIGEST, pp 582-585). For example, in conventional 25-inch CRTs, both lateral characteristics and NS characteristics are approximately 10 xcexcm. However, after the shadow mask is added, the lateral characteristics become 30 xcexcm and NS characteristics become 25 xcexcm. Typically, both characteristics deteriorate.
Up to now, there has been some attempts to improve the characteristics of the internal magnetic shield with the construction shown in FIG. 14. For example, the top of the short sides 142 of the internal magnetic shield is cut to form a V-shaped cut 144 as shown in FIG. 14, and optimization is performed by changing the depth or width of the cut 144.
The characteristics more greatly change when the depth of the V-shaped cut is changed, than when the width or the like is changed. FIG. 15 shows relation between the landing deviation and the depth of the V-shaped cut. As shown in FIG. 15, the deeper the cut is, the more improved the characteristics is. However, the NS characteristics rarely change. When the depth of the V-shaped cut is changed from 0 mm to 150 mm, the lateral characteristics change by 10 xcexcm, but the NS characteristics rarely change.
By the optimization of the V-shaped cut, the landing deviation caused by an external magnetic field equivalent to the terrestrial magnetism has been improved as follows:
(lateral characteristics, NS characteristics)=(20 xcexcm, 23 xcexcm)
However, improvement to both characteristics have not been achieved yet.
Also, the lateral characteristics and NS characteristics are in a tradeoff relationship in which the change rates of the characteristics are almost the same and the directions but reversed. This renders it more difficult to improve both the characteristics at the same time.
It is therefore an object of the present invention to provide a cathode ray tube that decreases the electron beam landing deviation caused by an external magnetic field such as from terrestrial magnetism and prevents the color on the screen from blurring or fading.
To fulfill the above object, the present invention is characterized by the magnetic field distribution inside the internal magnetic shield in the vicinity of the deflection coil and by the magnetic field distribution in the vicinity of the shadow mask.
To fulfill the above object, the magnetic field distribution on the trajectory of electron beams for displaying the circumferential portion of an image on the CRT screen is important. This portion corresponds to upper and lower areas each occupying approximately 20% of the area of a plane at the entrance of the internal magnetic shield.
It has been found that to improve the NS characteristics, the distribution of the vertical magnetic field (represented by xe2x80x9cByxe2x80x9d. Here, the term xe2x80x9cverticalxe2x80x9d indicates a direction along the vertical scanning direction) should be modified. More specifically, as shown in FIG. 16, a prominent effect is produced when the By component in the vicinity of the deflection coil and the By component in the vicinity of the shadow mask are oriented in opposite directions (plus and minus directions). Note that in FIG. 16, the magnetic field By is represented by a relative value. With this construction, it is possible to deviate the trajectory of an electron beam at the entrance of the internal magnetic shield to a direction opposite to a direction of deviation of the trajectory generated in the vicinity of the mask, offsetting a force that is applied to the electron beam perpendicular to the electron beam trajectory, and decrease the landing deviation of the electron beam.
To orient the By component in the vicinity of the deflection coil to the minus direction, the inventors improved the internal magnetic shield as follows.
(1) The inventors devised the shape of the internal magnetic shield so that the amount of magnetic flux absorbed at both ends (in the embodiment, the long sides) in the vertical scanning direction at the entrance of the electron beam in the vicinity of the deflection coil is larger than that at both ends (in the embodiment, the short sides) in the horizontal scanning direction.
(2) The inventors changed the effective permeability so that the amount of magnetic flux absorbed at both ends in the vertical scanning direction at the entrance of the electron beam in the vicinity of the deflection coil is larger than that at both ends in the horizontal scanning direction.
The effective permeability can be changed, for example, by forming both ends in the vertical scanning direction at the entrance of the electron beam in the vicinity of the deflection coil using a material having substantially high effective permeability, and forming both ends in the horizontal scanning direction using a material having substantially low effective permeability.
As described above, the present invention decreases the electron beam landing deviation by changing the magnetic field distribution inside the internal magnetic shield in the vicinity of the deflection coil and the magnetic field distribution in the vicinity of the shadow mask.